The range resolution achievable using radar techniques is determined by the signal bandwidth which can be generated and detected. The wider the bandwidth the better (smaller) the resolution. Current techniques provide a maximum of about 1 GHz bandwidth which corresponds to about 15 cm range resolution. Although systems using impulse radar techniques can provide more bandwidth and thus higher range resolution, they cannot be used as coherent radar systems and thus cannot discriminate between moving and stationary targets. Furthermore in impulse radar it is difficult to control the spectrum shape, which causes range sidelobes resulting in false target detections. Also, because impulse radar operates at low frequencies, it is impossible to transmit a narrow beam which means that such systems are unsuitable for long range radar in which angular discrimination is required.
In some applications high range resolution detection and motion measurement is required. This would be necessary if a radar system was to detect a target which is moving relatively with respect to a background since the detectable amplitude of a return signal from the target would be swamped by the return signal from the background. Examples of such applications include detecting an aircraft against a background of terrain and detecting a particular moving object among a number of moving objects.
It is an object of the invention to provide a radar system having an improved radar resolution. According to a first aspect the invention provides radar apparatus comprising waveform generator means for generating a signal at an intermediate frequency, a coupler for dividing the signal into two divided signals, a mixer for mixing the two divided signals together to generate an output signal and transmission means for transmitting the output signal as a radar signal in which the output signal has a wider bandwidth than the signal at the intermediate frequency.
Preferably the waveform generator means generates a wideband signal at the intermediate frequency. Preferably the intermediate frequency is at 1 GHz or higher. In one embodiment the intermediate frequency is less than 1 GHz, for example 500 MHz.
Preferably the waveform generator means comprises a waveform generator and a vector modulator. The vector modulator may be driven by a local oscillator at a suitable intermediate frequency.
Preferably the waveform generator comprises a two channel digital waveform generator. Preferably the two waveform channels are fed to in-phase and quadrature inputs of the vector modulator to produce a waveform at the intermediate frequency with twice the bandwidth of each waveform channel.
Preferably the output of the vector modulator is filtered to remove spurious signals and amplified by an amplifier before entering the coupler. Preferably, the gain of the amplifier and the coupling value of the coupler may be chosen to give suitable input levels to the mixer.
Preferably a path between a first output of the coupler and a first input of the mixer and a path between a second output of the coupler and a second input of the mixer are of equal length, or of equal phase length.
Preferably the signals which are fed into the mixer have power levels which are substantially different. One may be 15 to 20 dB lower in power level than the other.
Preferably the output signal from the mixer is filtered to remove spurious signals. Preferably the frequency of the output signal is twice the intermediate frequency of the signal. Preferably the bandwidth of the output signal is twice the bandwidth of the intermediate frequency signal.
Use of the word xe2x80x9cfrequencyxe2x80x9d in relation to a signal or waveform may be reference to the centre frequency of the signal or waveform.
The output signal may be transmitted from an antenna as a radar signal or may be sent to one or more additional coupler and mixer combinations to increase the bandwidth further. The or each additional mixer may produce a new output signal. Suitable amplification and filtering may be provided in relation to each coupler and mixer combination.
Preferably there are two mixers. In an embodiment in which there are two mixers which each double the frequency and bandwidth, the bandwidth of the intermediate signal can be increased by a factor of four. More than two mixers may be present.
Preferably the apparatus includes a sufficient number of mixers to provide a range resolution which is less than 10 cm. It may be in the order of a few cm.
Preferably the waveform is a frequency modulated waveform, for example frequency modulated continuous wave (FMCW) or a within pulse chirp. The output signal may also have FMCW modulation or a within pulse chirp.
The output signal which is ultimately produced may be transmitted from an antenna or may first be translated to a different centre frequency using a conventional mixer.
Preferably the apparatus is incorporated into a radar detection system comprising the radar apparatus and means for receiving and processing radar signals.
Preferably the means for receiving radar signals is a correlation receiver, in which the received signal (reflected from a target) is correlated with a reference signal. Conveniently the reference signal is a delayed sample of the output signal which was transmitted.
Correlation may be performed by mixing the received signal and the reference signal in a wideband mixer and integrating the mixer output for a time corresponding to the length of the transmitted waveform. Conveniently this integration may be performed by a low pass filter with a suitable cut-off frequency.
Preferably the correlation receiver retains the phase information of the received signal by performing the correlation in In-phase (I) and Quadrature (Q) channels. Alternatively an offset frequency may be employed so that the correlation mixer output is at an intermediate frequency, and the subsequent down-conversion to baseband may be performed in I and Q channels.
Preferably the radar system uses coherent processing techniques. It may use Doppler techniques to measure the speed or velocity of a target. It may measure the speed or velocity of the target relative to that of a background.
According to a second aspect the invention provides a method of generating a radar signal comprising the steps of:
generating a signal having an intermediate frequency;
dividing the signal into two divided signals;
mixing the divided signals to generate an output signal having a wider bandwidth than the
intermediate frequency signal; and
transmitting the output signal as a radar signal.
Preferably mixing of the divided signals is performed by a mixer. The output signal produced by the mixer may be transmitted from an antenna as a radar signal or may undergo at least one additional mixing step. The or each additional mixing step may produce a new output signal. Preferably there are two mixing steps. There may be more than two mixing steps. The or each mixing step may double the bandwidth of the waveform. The or each mixing step may double the frequency of the waveform.
Preferably the method is used in a radar detection method comprising the steps of: generating the radar signal, receiving a reflected radar signal and processing the reflected radar signal.
Preferably a received signal may be correlated with a reference signal. The received signal may be match filtered against the reference signal. Conveniently the reference signal is identical to the output signal which was transmitted. Received signals may be match filtered against the reference signal in in-phase and quadrature channels.
The invention may be used in a pulse radar system, although it could be used on a CW signal which has suitable wideband modulation.
The invention may be applied to short range and long range radar detection. It may be used in ground penetration radar or in a radar for the detection, imaging or tracking of moving objects.